MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) are a common component in integrated circuits fabricated on a semiconductor wafer. The current characteristic of a MOSFET when the MOSFET is turned on is as follows: EQU I.sub.D =k(V.sub.GS -V.sub.t).sup.2
with I.sub.D being the drain current of the MOSFET, k being a constant that depends on the size and fabrication parameters of the MOSFET, V.sub.GS being the gate to source voltage of the MOSFET, and V.sub.t being the threshold voltage of the MOSFET, as known to one of ordinary skill in the art of electronics.
For better circuit performance, higher speed and thus higher drain current is desired when the MOSFET is turned on. Thus, when the MOSFET is turned on, a threshold voltage V.sub.t with lower magnitude is desired. On the other hand, for lower steady state power dissipation when the MOSFET is turned off, a threshold voltage with higher magnitude is desired.
Higher device speed and lower power dissipation require opposite constraints on the threshold voltage of a MOSFET. In prior art MOSFETs, the threshold voltage is typically the same for when the MOSFET is turned on and for when the MOSFET is turned off. Therefore, a trade-off between higher device speed and lower steady state power dissipation is considered with the prior art MOSFET device.
However, both higher device speed and lower steady state power dissipation are desired for high performance integrated circuits. Thus, a MOSFET type device is desired that may be designed both for higher device speed when the MOSFET is turned on and for lower steady state power dissipation when the MOSFET is turned off.